Barack Obama has given the strongest indication to date that he holds reservations about the Keystone XL tar sands pipeline, saying the project would not create many jobs and could raise gasoline prices.

In an interview with the New York Times, the president disputed a main justification for the pipeline – its economic benefits – and reaffirmed he would reject the project if it expanded carbon pollution.

The comments were seen by campaigners as evidence that Obama, in the wake of last month's landmark climate change speech, was leaning towards rejecting the project.

Obama has been under growing pressure from campaigners, party donors, and Democrats in Congress to reject the pipeline, which would expand production from Canada's tar sands.

He adopted some of their arguments in his comments on Saturday, knocking down pipeline supporters' claims of a big jobs boost, saying Keystone would register little more than a "blip" on the employment rolls.

"Republicans have said that this would be a big jobs generator. There is no evidence that that's true," Obama said in the interview.

"The most realistic estimates are this might create maybe 2,000 jobs during the construction of the pipeline – which might take a year or two – and then after that we're talking about somewhere between 50 and 100 jobs in a economy of 150 million working people."

The president – without prompting by the reporter – then noted that the project would not bring down gas prices, and might even raise them.

However, Obama said – as he did in his climate change address last month – that his decision on the pipeline would be based on the pipeline's effects on climate change.

"I'm going to evaluate this based on whether or not this is going to significantly contribute to carbon in our atmosphere. And there is no doubt that Canada at the source in those tar sands could potentially be doing more to mitigate carbon release."

It was the second time in just over a month that Obama has cited the environmental effects of the pipeline, following his mention of the project in his sweeping climate change address.

But the president once again was not explicit about whether he thought the pipeline would accelerate climate change.

Tar sands crude is far more carbon intensive than conventional oil, and campaigners have cast the pipeline as a test of Obama's environmental commitment.

The State Department ruled last March that the pipeline would not increase greenhouse gas emissions – essentially arguing the tar sands would be mined anyway.

But the Environmental Protection Agency in a strongly worded letter called for a more detailed study one month later.

The administration is expected to make its decision towards the end of the year.